I can answer that Tig but it's a long story. 
Going back two or three hundred years the areas east of a city and its main river were always the poorest and least desirable. That was because they housed the 'stink industries' - the tanning and glue factories and the processing plants, and the docks, so that any unpleasant smells would be carried out east on the wind into the wilderness, while the rich people could afford to live in the west, where the smells drifted away from them, and not towards them. So the East End and south Essex was always traditionally less desirable than places in the west. All the shit (metaphorically and I suspect literally) would have been carried on barges on the Thames, eastwards for disposal.
Later, many working class people from the East End were moved out to new build towns (Harlow, Basildon, Dagenham etc) after the war as their areas had been blitzed, and they had a bad rep for being rough anyway (the docks etc) so some areas that had been rural became full of East and North London overspill. People would move out from the East End and settled in the countryside towns they knew as children - places like Clacton and Southend, with their severance pay from the docks, and the print etc, when they all closed down. By and large they did pretty well for themselves, compared to their East Ender parents, although they still had working class sensibilities. Hence why certain towns carry a comical/negative stereotype of Flash Harry upstarts with too much money and no taste.
Then, in the eighties, during the Thatcher and Loadsmoney era, and when the city was deregulated, lots of working class kids got on the train aged sixteen and headed to London for a job, and if they were from Essex they got off at Liverpool Street, or Fenchurch Street and ended up working in banking or insurance, on the bottom rung. If they were bright they got promoted and ended up making shedloads of money, and for the first time they were competing on equal terms with people who had public school and university educations. Hence the 'Barrow Boy' label. The classic 80's city trader image of the wide boy with his red braces and his bottle of champagne and his Rolex.
I think the upper middle class types and the media intelligensia secretly despised them for succeeding and showing off and set about making them a laughing stock.
Plus, there was 'Basildon Man'. He was a construct of some political think tank, who said that the typical Tory voter was a bloke in his thirties from Basildon, driving a Mondeo. (or something) Because typically he would be someone who had achieved social mobility through buying his council house, maybe a few shares in BT, starting his own business and grafting hard, and being proud of his endeavours. (see the Loadsamoney character.)
But it is a huge county, with all types of people from all backgrounds, and people should remember that that the jokey stereotype represents just a drop in the ocean in real terms. I come from Kent originally and they are not a bit different there! People in south Essex have a lot in common with people from North Kent - the towns along either side of the estuary tend to mirror one another.
So I do get a bit irritated when I hear someone from somewhere like Dartford or Bexley banging on spitefully about 'Essex Girls', when most parts of north Essex are full of horsey girls with nice accents and Joules clothing.